diff options
Diffstat (limited to 'compiler/GHC/Tc/Validity.hs')
| -rw-r--r-- | compiler/GHC/Tc/Validity.hs | 30 |
1 files changed, 24 insertions, 6 deletions
diff --git a/compiler/GHC/Tc/Validity.hs b/compiler/GHC/Tc/Validity.hs index 5ac3377a33..949eb90d53 100644 --- a/compiler/GHC/Tc/Validity.hs +++ b/compiler/GHC/Tc/Validity.hs @@ -520,6 +520,8 @@ typeOrKindCtxt (DataKindCtxt {}) = OnlyKindCtxt typeOrKindCtxt (TySynKindCtxt {}) = OnlyKindCtxt typeOrKindCtxt (TyFamResKindCtxt {}) = OnlyKindCtxt +-- These cases are also described in Note [No constraints in kinds], so any +-- change here should be reflected in that note. typeOrKindCtxt (TySynCtxt {}) = BothTypeAndKindCtxt -- Type synonyms can have types and kinds on their RHSs typeOrKindCtxt (GhciCtxt {}) = BothTypeAndKindCtxt @@ -543,7 +545,6 @@ typeLevelUserTypeCtxt ctxt = case typeOrKindCtxt ctxt of -- | Returns 'True' if the supplied 'UserTypeCtxt' is unambiguously not the -- context for a kind of a type, where the arbitrary use of constraints is -- currently disallowed. --- (See @Note [Constraints in kinds]@ in "GHC.Core.TyCo.Rep".) allConstraintsAllowed :: UserTypeCtxt -> Bool allConstraintsAllowed = typeLevelUserTypeCtxt @@ -931,10 +932,9 @@ checkConstraintsOK :: ValidityEnv -> ThetaType -> Type -> TcM () checkConstraintsOK ve theta ty | null theta = return () | allConstraintsAllowed (ve_ctxt ve) = return () - | otherwise - = -- We are in a kind, where we allow only equality predicates - -- See Note [Constraints in kinds] in GHC.Core.TyCo.Rep, and #16263 - checkTcM (all isEqPred theta) (env, TcRnConstraintInKind (tidyType env ty)) + | otherwise -- We are unambiguously in a kind; see + -- Note [No constraints in kinds] + = failWithTcM (env, TcRnConstraintInKind (tidyType env ty)) where env = ve_tidy_env ve checkVdqOK :: ValidityEnv -> [TyVarBinder] -> Type -> TcM () @@ -945,7 +945,25 @@ checkVdqOK ve tvbs ty = do no_vdq = all (isInvisibleForAllTyFlag . binderFlag) tvbs ValidityEnv{ve_tidy_env = env, ve_ctxt = ctxt} = ve -{- +{- Note [No constraints in kinds] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +GHC does not allow constraints in kinds. Equality constraints +in kinds were allowed from GHC 8.0, but this "feature" was removed +as part of Proposal #547 (https://github.com/ghc-proposals/ghc-proposals/pull/547), +which contains further context and motivation for the removal. + +The lack of constraints in kinds is enforced by checkConstraintsOK, which +uses the UserTypeCtxt to determine if we are unambiguously checking a kind. +There are two ambiguous contexts (constructor BothTypeAndKindCtxt of TypeOrKindCtxt) +as written in typeOfKindCtxt: + - TySynCtxt: this is the RHS of a type synonym. We check the expansion of type + synonyms for constraints, so this is handled at the usage site of the synonym. + - GhciCtxt: This is the type in a :kind command. A constraint here does not cause + any trouble, because the type cannot be used to classify a type. + +Beyond these two cases, we also promote data constructors. We check for constraints +in data constructor types in GHC.Tc.Gen.HsType.tcTyVar. + Note [Liberal type synonyms] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If -XLiberalTypeSynonyms is on, expand closed type synonyms *before* |